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AR# 52592

PlanAhead - The SOFT attribute is not supported, "CRITICAL WARNING: [Designutils 20-31] Unrecognized symbol SOFT"


For LOC constraint, there is a SOFT attribute:

INST instance_name LOC=location :location {SOFT };

If SOFT is specified, PAR may place the constraint elsewhere if better results can be obtained at a location outside the bounding box. Examples of LOC constraints used to specify a range are given below.

The SOFT attribute is supported in the ISE design tools, but not in the PlanAhead tool:

CRITICAL WARNING: [Designutils 20-31] Unrecognized symbol SOFT [C:/Project/bft/bft.srcs/constrs_2/imports/Sources/bft_full.ucf:4]


The Soft attribute for LOC will not be available in PlanAhead (or in Vivado XDC). You should use Pblocks to get a "soft LOC" (i.e., range).

In the PlanAhead tool, you could work around this issue by adding the soft LOC to a UCF file, and include the ucf file using the -uc option in the NGDBUILD other command line options.

AR# 52592
Date Created 10/25/2012
Last Updated 11/08/2013
Status Active
Type General Article
  • FPGA Device Families
  • ISE Design Suite - 14